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221.
222.
A rapid, selective, sensitive and simple fluorescence method was developed for the direct determination of celecoxib in capsules. The capsules were emptied, pulverized and dissolved in either ethanol or acetonitrile, sonicated and filtered. Direct fluorescence emission was measured at 355±5 nm (exciting at 272 nm). The method was fully validated and the recoveries were excellent, even in presence of excipients. 相似文献
223.
Binding of single nucleotides to H+-ATP synthases observed by fluorescence resonance energy transfer
Steigmiller S Zimmermann B Diez M Börsch M Gräber P 《Bioelectrochemistry (Amsterdam, Netherlands)》2004,63(1-2):79-85
F(0)F(1)-ATP synthases couple proton translocation with the synthesis of ATP from ADP and phosphate. The enzyme has three catalytic nucleotide binding sites, one on each beta-subunit; three non-catalytic binding sites are located mainly on each alpha-subunit. In order to observe substrate binding to the enzyme, the H(+)-ATP synthase from Escherichia coli was labelled selectively with the fluorescence donor tetramethylrhodamine (TMR) at position T106C of the gamma-subunit. The labelled enzymes were incorporated into liposomes and catalysed proton-driven ATP synthesis. The substrate ATP-Alexa Fluor 647 was used as the fluorescence acceptor to perform intermolecular fluorescence resonance energy transfer (FRET). Single molecules are detected with a confocal set-up. When one ATP-Alexa Fluor 647 binds to the enzyme, FRET can be observed. Five stable states with different intermolecular FRET efficiencies were distinguished for enzyme-bound ATP-Alexa Fluor 647 indicating binding to different binding sites. Consecutive hydrolysis of excess ATP resulted in stepwise changes of the FRET efficiency. Thereby, gamma-subunit movement during catalysis was directly monitored with respect to the binding site with bound ATP-Alexa Fluor 647. 相似文献
224.
A diastereoselective [4+2] annulation of vinyl carbodiimides with chiral N-alkyl imines has been developed to access the stereochemically rich tricyclic core of the batzelladine alkaloids. Its application to the asymmetric synthesis of batzelladine D permitted the use of long-range, directed hydrogenation and stereoselective intramolecular iodoamination as additional key steps to establish the remaining stereocenters within the natural product with excellent stereocontrol. 相似文献
225.
Aarnts MP Wilms MP Peelen K Fraanje J Goubitz K Hartl F Stufkens DJ Baerends EJ Vlcek A 《Inorganic chemistry》1996,35(19):5468-5477
Ru(SnPh(3))(2)(CO)(2)(iPr-DAB) was synthesized and characterized by UV-vis, IR, (1)H NMR, (13)C NMR, (119)Sn NMR, and mass (FAB(+)) spectroscopies and by single-crystal X-ray diffraction, which proved the presence of a nearly linear Sn-Ru-Sn unit. Crystals of Ru(SnPh(3))(2)(CO)(2)(iPr-DAB).3.5C(6)H(6) form in the triclinic space group P&onemacr; in a unit cell of dimensions a = 11.662(6) ?, b = 13.902(3) ?, c = 19.643(2) ?, alpha = 71.24(2) degrees, beta = 86.91(4) degrees, gamma = 77.89(3) degrees, and V = 2946(3) ?(3). One-electron reduction of Ru(SnPh(3))(2)(CO)(2)(iPr-DAB) produces the stable radical-anion [Ru(SnPh(3))(2)(CO)(2)(iPr-DAB)](*-) that was characterized by IR, and UV-vis spectroelectrochemistry. Its EPR spectrum shows a signal at g = 1.9960 with well resolved Sn, Ru, and iPr-DAB (H, N) hyperfine couplings. DFT-MO calculations on the model compound Ru(SnH(3))(2)(CO)(2)(H-DAB) reveal that the HOMO is mainly of sigma(Sn-Ru-Sn) character mixed strongly with the lowest pi orbital of the H-DAB ligand. The LUMO (SOMO in the reduced complex) should be viewed as predominantly pi(H-DAB) with an admixture of the sigma(Sn-Ru-Sn) orbital. Accordingly, the lowest-energy absorption band of the neutral species will mainly belong to the sigma(Sn-Ru-Sn)-->pi(iPr-DAB) charge transfer transition. The intrinsic strength of the Ru-Sn bond and the delocalized character of the three-center four-electron Sn-Ru-Sn sigma-bond account for the inherent stability of the radical anion. 相似文献
226.
Antolín R Borge G Posada T Raposo JC Seara I Etxebarria N 《Journal of AOAC International》2004,87(5):1218-1223
A validation study was carried out in order to evaluate the efficiency of inductively coupled plasma-optical emission spectrometry (ICP-OES) for the analysis of minor elements (manganese, chromium, copper, iron, and titanium) in aluminium alloys. Aluminium casting samples were obtained by adding compressed powder compacts of each alloying element and aluminium (minitablets) to aluminum baths in a laboratory crucible furnace. Digestion of solid samples was performed using concentrated HCI and H202 35% (v/v) previous to analysis by ICP-OES without any matrix separation. This solution-based method was validated considering direct current arc spectrometry as the reference method based on direct analysis without any pretreatment of the solid samples considered. Univariate statistical procedures were carried out, for which precision <3% and trueness of the analytical results were taken into account. 相似文献
227.
228.
F. Román S. Montserrat J. M. Hutchinson 《Journal of Thermal Analysis and Calorimetry》2007,87(1):113-118
The procedure for
the fabrication of epoxy-based polymer layered silicate nanocomposites is
important in respect of the nanostructure that is developed. To further our
understanding of this, the influence of an organically modified clay (montmorillonite,
MMT) on the curing kinetics of an epoxy resin has been studied by differential
scanning calorimetry. Clay loadings of 10 and 20 mass% are used, and isothermal
as well as dynamic cures have been investigated. For both cure schedules the
effect of the MMT is to advance the reaction. Kinetic analysis yields values
for the activation energy, but shows that the reaction cannot be described
simply by the usual autocatalytic equation. The glass transition of the cured
nanocomposites is lower than that for the cured neat resin, a result that
is attributed to homopolymerisation taking place in addition to the epoxy–amine
reaction. 相似文献
229.
Kótai László Gács István Kazinczy Béla Sajó István E. Sreedhar B. 《Transition Metal Chemistry》2003,28(3):292-295
The theoretical investigation on the distribution of chemical species obtainable as a function of the temperature of the aqueous solutions of metal complexes containing a basic ligand, altogether with the solubility relationship of the compounds to be formed, offers a simple means of planning the preparation of various types of complexes, double and single salts of the central metal cation and onium type compounds of protonated ligand of the central ion and the anion. 相似文献
230.